Physics Lesson 15.4.5 - Short Circuits

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Welcome to our Physics lesson on Short Circuits, this is the fifth lesson of our suite of physics lessons covering the topic of Electric Circuits. Series and Parallel Circuits. Short Circuits, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Short Circuits

It is a known fact that the electric current chooses the easiest path to flow. This path is not meant to be the shortest one. When we bypass a resistor through an extra conducting wire added in that part of the circuit, this action brings the formation of a short circuit, which is a circuit with a very low resistance (just the resistance of the conductor and that of the source), and which brings an increase of the current in the circuit. In other words, a short circuit "cancels" the effect of the resistance it bypasses. Look at the figure.

Example 3

An electric circuit composed by three resistors R₁ = 18 Ω, R₂ = 12 Ω and R₃ = 9 Ω connected in parallel to a 24 V battery is shown in the figure below.

The resistance of wire is Rw = 0.5 Ω and that of battery is r = 1 Ω.

Calculate the current in the main branch if:

1. the circuit is in the actual state
2. the first resistor cuts in two pieces which separate from each other leaving a gap
3. An additional piece of wire is used to bypass the first resistor as shown in the figure in theory.

Solution 3

a. First, we have to calculate the equivalent resistance of the parallel branch. We have,

Thus,

The total resistance in the circuit is

Therefore, the current in the main branch is

b. If the first resistor is faulty and cuts in two pieces, no current flows through it. As a result, only the other two resistors are in use. We use the same procedure as in part (a) to calculate the current in the main branch, i.e.

Thus,

The total resistance in the circuit is

Therefore, the current in the main branch is

c) If we connect a piece of wire to bypass the first resistor, all the other resistors are bypassed as well. Therefore, only the resistance of wire and battery are to be considered. We have

Therefore, the current in the main branch is

As you see, when a short circuit does occur, the current in the main branch is much higher than usual. Therefore, we must avoid such situations as a short circuit may cause damage to electrical appliances. To protect the appliances from such unexpected situations, producers usually install some inexpensive devices called fuses, which are connected in series with them. A good fuse holds a maximum current up to 0.5 A higher than the operating current of the appliance. Thus, for example, if an appliance operates at 8A current, a fuse with a maximum operating capacity of 8.5 A is installed before the appliance. In cases of short circuits when the current in the circuit becomes much higher than the operating current of the appliance, the fuse connected in series to it breaks first, protecting in this way the corresponding appliance as the current flow stops.

You have reached the end of Physics lesson 15.4.5 Short Circuits. There are 5 lessons in this physics tutorial covering Electric Circuits. Series and Parallel Circuits. Short Circuits, you can access all the lessons from this tutorial below.

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